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From: Richard Stallman <rms@gnu.org>
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Subject: Re: Warning: `mapcar' called for effect; use `mapc' or `dolist'
Date: Wed, 31 Oct 2007 03:46:58 -0400
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Xref: news.gmane.org gmane.emacs.gnus.general:65500 gmane.emacs.devel:82193

[I sent this message a week ago but did not get a response.]

    >   > (defun gnus-group-highlight-line ()
    >   >   "Highlight the current line according to `gnus-group-highlight'."
    [...]
    > It seems that `mailp' is not used in that function. 

    `mailp' is used implicitly through the variable `gnus-group-highlight'
    which is let-bound to `list'.  `list' is used in the body of the
    let-form.

    > Adding a use for it makes the warning go away...

If the byte compiler warns about that case, it is being too ambitious.
Thus, if the value of an expression is stored into a variable, the
compiler should consider the value "used".

So if this warning really happens, it is a bug.
Can it be reproduced?